Bottom Line:
We propose and experimentally verify a cooling limit for a quantum channel going through an incoherent environment.The environment consists of a large number of independent non-interacting and non-interfering elementary quantum systems--qubits.The limit specifies when the single-qubit channel is quantum, i.e. it preserves entanglement.

ABSTRACTWe propose and experimentally verify a cooling limit for a quantum channel going through an incoherent environment. The environment consists of a large number of independent non-interacting and non-interfering elementary quantum systems--qubits. The qubits travelling through the channel can only be randomly replaced by environmental qubits. We investigate a conditional cooling limit that exploits an additional probing output. The limit specifies when the single-qubit channel is quantum, i.e. it preserves entanglement. It is a fundamental condition for entanglement-based quantum technology.

f4: The data from Fig. 3, where each colour denotes a certain value of PL.Square points represent the states, for which the entanglement was preserved only conditionally.

Mentions:
In Figs 3 and 4, the measured data are shown. Figure 3 best illustrates the parametric space spanned by PS, PL, pT. The blue surface represents the conditional bound for separability (6), while the thick grey lines belong to the surface representing the unconditional bound (2). Therefore, spherical data points below the blue surface represent quantum states, where the entanglement is lost. Spherical points above the surface are the states, which remain entangled unconditionally. Cube points are the states between the two conditions—separable unconditionally, but entangled using the auxiliary projection.

f4: The data from Fig. 3, where each colour denotes a certain value of PL.Square points represent the states, for which the entanglement was preserved only conditionally.

Mentions:
In Figs 3 and 4, the measured data are shown. Figure 3 best illustrates the parametric space spanned by PS, PL, pT. The blue surface represents the conditional bound for separability (6), while the thick grey lines belong to the surface representing the unconditional bound (2). Therefore, spherical data points below the blue surface represent quantum states, where the entanglement is lost. Spherical points above the surface are the states, which remain entangled unconditionally. Cube points are the states between the two conditions—separable unconditionally, but entangled using the auxiliary projection.

Bottom Line:
We propose and experimentally verify a cooling limit for a quantum channel going through an incoherent environment.The environment consists of a large number of independent non-interacting and non-interfering elementary quantum systems--qubits.The limit specifies when the single-qubit channel is quantum, i.e. it preserves entanglement.

ABSTRACTWe propose and experimentally verify a cooling limit for a quantum channel going through an incoherent environment. The environment consists of a large number of independent non-interacting and non-interfering elementary quantum systems--qubits. The qubits travelling through the channel can only be randomly replaced by environmental qubits. We investigate a conditional cooling limit that exploits an additional probing output. The limit specifies when the single-qubit channel is quantum, i.e. it preserves entanglement. It is a fundamental condition for entanglement-based quantum technology.